1. Field of the Invention
The present invention relates to a cleaning technique employed in a process of fabricating a semiconductor device, in particular, to a substrate cleaning technique for cleaning a substrate having a surface on which a metal material is exposed.
2. Description of the Related Art
Recently, in view of a higher operation speed of logic devices, a copper wiring having low resistance, excellent in resistance against electromigration, has been widely used. Since the copper is hard to process by dry etching, a wiring pattern is formed by a Damascene process utilizing CMP (Chemical Mechanical Polishing) in the case where a copper wiring is to be formed. However, since the CMP is a process for polishing a copper film with polishing particles, a large amount of particles or metal contamination are adhered to a surface of a wafer as the result of CMP. Therefore, it is important to thoroughly clean the surface of the wafer after completion of CMP.
A cleaning process after CMP is normally conducted according to the process as shown in FIG. 1. Specifically, after conducting a CMP (Chemical Mechanical Polishing) process on a substrate having a Cu-exposed portion (hereinafter, this process is abbreviated as “Cu-CMP”) (Step S101), cleaning for removing particles is first conducted (Step S102). This cleaning is normally conducted by brush scrub cleaning using an alkaline cleaning solution. Subsequently, cleaning for removing metal contamination is conducted (Step S103). This second cleaning is conducted with an organic acid cleaning solution including, for example, oxalic acid or citric acid. Then, after rinsing a wafer with pure water (Step S104), the substrate is dried to complete the cleaning process (Step S105).
The above-described process will be further described with reference to cross-sectional views showing the respective steps. FIG. 2A to FIG. 2D are cross-sectional views showing the substrate while the process in FIG. 1 is being conducted. First, as shown in FIG. 2A, a wiring groove is formed on a surface of an insulating film 101. Then, a barrier metal film 102 and a copper film 103 are sequentially formed on the insulating film 101.
Subsequently, as shown in FIG. 2B, the copper film 103 is polished by CMP so as to leave the copper film 103 only in the wiring groove. After completion of CMP, a large amount of particles 104 and metal contamination 105 are adhered onto the surface of the substrate.
Next, as shown in FIG. 2C, brush scrub cleaning is first conducted by using an alkaline cleaning solution so as to remove these particles. As a result, the particles 104 are removed.
Subsequently, as shown in FIG. 2D, the metal contamination 105 is removed by oxalic acid, citric acid or the like. At this moment, however, slits 110 are sometimes generated between the barrier metal film 102 and the copper film 103. Although a generation mechanism of the slits 110 is not completely elucidated, it is believed that these slits are generated due to a kind of battery effect. In the case where such slits are generated, there arises a problem in that the degradation in reliability (resistance against electromigration or the like) of the copper wiring is induced. In addition, when a film is to be formed in the wiring layer, the flatness of the film is lost. It is difficult to completely prevent the generation of such slits even if an anticorrosive is added to an organic acid cleaning solution.
The slits may be generated in a similar manner in a process of forming a through hole on the copper wiring. FIG. 3 shows a state where the slits are generated when a through hole is formed by dry etching after formation of an interlayer insulating film on the copper wiring. A copper wiring having a structure including a laminate of the barrier metal film 102 and the copper film 103 is provided in the wiring groove formed on the insulating film 101. Then, a diffusion barrier film 107 and an interlayer insulating film 108 are formed thereon. A through hole 109 reaching the copper wiring is formed by dry etching. After formation of the through hole 109, a process for removing a resist film or a process for cleaning the inside of the through hole are conducted. In these processes, the slits 110 are generated due to the action of a resist stripping solution or the like. With the increased fineness of an element in these days, a socalled borderless wiring has been widely utilized. The borderless wiring has a wiring width equal to a through hole diameter. In the case where such a structure is employed, even with slight misalignment of a mask, a boundary face between the barrier metal film 102 and the copper film 103 is exposed, increasing the possibility of generating the above-described slits.
In each of the above-described processes, not only the generation of the slits, but also an increase in resistance due to increased roughness of the surface of the copper film and a drop in resistance against electromigration become problems in some cases. These problems arise because an oxide film present on the surface of the copper film is dissolved in acid to generate minute concave and convex portions on the surface of the copper film.
The problems of the generation of the slits inherent to the use of a cleaning solution or a stripping solution have been described above. The problem induced by using the cleaning solution is not limited to the generation of the slits; conventional cleaning also involves a problem of damaging an underlayer. FIG. 4 shows a state after formation of a contact hole reaching source/drain regions 122 of a transistor. In this case, a field-effect transistor 121 is provided on a silicon substrate 120. On both sides of the field effect transistor 120, source/drain regions 122 are respectively formed. An interlayer insulating film 123 is formed on the thus constituted transistor. A contact hole 125 reaching the source/drain regions 122 is provided through the interlayer insulating film 123. In a process for providing the contact hole 125, metal contamination 124 is adhered to an inner wall of the contact hole 125. This metal contamination 124 is generated during dry etching, and is brought to be adhered onto the inner wall of the contact hole. In order to remove the metal contamination 124, cleaning with a mixed solution of hydrogen peroxide and acid has been conventionally conducted. If such cleaning is performed, however, an oxide film is formed on a bottom face of the contact hole, which sometimes leads to an increase in resistance. It is possible to conduct the cleaning using a hydrofluoric acid cleaning solution instead of the above mixed solution. In such a case, however, side etching on sidewalls of the contact hole proceeds. As a result, a desired hole shape cannot be obtained in some cases. As described above, if acid cleaning for removing the metal contamination is conducted in the semiconductor device fabrication process, there arise various problems such as the generation of slits in the metal film or the damages to the underlayer.
As a technique for removing the contamination comprising a metal and a metal oxide/hydroxide without damaging the metal wiring, Japanese Patent Laid-Open Publication No. Hei. 10-41262 discloses a cleaning method using a neutral or acid cleaning solution whose oxidation-reduction potential is controlled by dissolving a hydrogen gas or the like therein. However, this conventional technique relates to a technique for removing contamination comprising iron or its oxide/hydroxide. This technique provides a cleaning method using a neutral or acid cleaning solution instead of a conventionally used strong acid cleaning solution so as not to damage a metal wiring when the contamination is to be removed. More specifically, as cleaning solutions, water containing dissolved hydrogen, which has an oxidation-reduction potential (with respect to a normal hydrogen electrode) of −300 to −400 mV and pH7, or an aqueous solution having pH 5.5, which is obtained by bubbling hydrogen in a carbon dioxide aqueous solution to be dissolved and the like are disclosed. However, although these cleaning solutions are effective for removing the metal contamination of specific metals such as iron metals, it is difficult to remove metal contamination of hardly removable metals such as copper, or particles containing the same metal as the metal film on which the particles are adhered. Moreover, in the method of bubbling and dissolving hydrogen in a carbon dioxide aqueous solution, a sufficient amount of hydrogen is not dissolved, so that the oxidation-reduction potential becomes relatively high. As a result, this method is not necessarily effective to deal with the problems such as the generation of slits or the surface roughness in a copper wiring formation process.